[amsat-bb] Re: fun with preamps

i8cvs domenico.i8cvs at tin.it
Mon Mar 15 17:31:31 PDT 2010

----- Original Message -----
From: "Bob- W7LRD" <w7lrd at comcast.net>
To: <amsat-bb at amsat.org>
Sent: Tuesday, March 16, 2010 12:28 AM
Subject: [amsat-bb] fun with preamps

> Hello
> Just installed a AG-25 (Icom preamp) on my 2M cp yagi. No preamp noise
Smeter=0, preamp on Smeter=7. Signals "appear" better after backing down the
volume with the preamp. I have 8-10 feet of LMR400uf from antenna to preamp,
50 feet of LMR400 to shack. I suppose I was expecting more "dramatic"
results, I realize the AG-25 isn't the greatest and best, but it works well
with the 910. Are these real world results for what I have. S7 noise level
seems a bit high.
> as always-thanks
> 73 Bob W7LRD

Hi Bob, W7LRD

No preamp noise Smeter=0, preamp on Smeter=7 means very small. Step 1
without preamplifier tune your receiver in SSB or CW on a very weak carrier
like a beacon and rotate your antenna into a null until you can barely hear
a bit at about 1000 Hz over the noise. Step 2 now connect your preamplifier
and see by hears if the Signal to Noise ratio S/N improves or not. If you
can hear better the 1000 Hz bit your preamplier is useful othervise S meter
= 7 means only or mostly the amplified preamplifier noise.
Your test can be more accurate if instead to use your hears you connect an
analog AC voltmeter at the audio output of your receiver.More is  the swing
of the AC voltmeter and better is the preamplifier i.e. lover is it's Noise

Have fun.

73" de

i8CVS Domenico

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